dvc repro: make the --pull flag work with --downstream flag

[ettadar]

Currently the --pull flag have no impact when used together with the --downstream flag. When working in a new repo, one must pull manually all the deps of stage A (if there is too much data handled with dvc to run dvc pull) before running dvc repro --downstream A. It would be great if dvc repro --downstream --pull A would automatically pull A deps.

Linked to #4870
Discord discussion: https://discord.com/channels/485586884165107732/485596304961962003/775412039321387008

[ettadar]

Thinking more about it it would be great if --pull would work with -s|--single-item too.